Ultrafast structural flattening motion in photoinduced excited state dynamics of a bis(diimine) copper(i) complex

Abstract

The ultrafast photoinduced structural change dynamics of a prototypical Cu(I) complex, namely, [Cu(dmp)₂]⁺ (dmp = 2,9-dimethyl-1,10-phenanthroline), is investigated based on the theoretical analysis of static and dynamical calculations at the all-atomic level. This work mainly focuses on the intriguing structural flattening features of [Cu(dmp)₂]⁺ occurring in the metal-to-ligand charge transfer singlet excited state (¹MLCT) on the sub-picosecond timescale. Our estimated time constant (∼675 fs) of this “flattening” motion is in good agreement with recent experimental values. The full-dimensional excited-state nonadiabatic dynamic simulation provides a direct view of the ultrafast photoinduced events of [Cu(dmp)₂]⁺, especially, the structural flattening mechanism on the S₁ state. Several molecular motions (such as Cu–N stretching, the motion of the substituted groups etc.) with distinguishable time scales are involved in the flattening dynamics. The Fourier transformation of the time-dependent oscillation of the Cu–N bond and the N–Cu–N bond angle provides consistent conclusions with the experimental spectrum analysis. These dynamics details imply that various nuclear motions are strongly coupled in the high-dimensional excited-state potential energy surface responsible for the geometrical evolution of [Cu(dmp)₂]⁺. This work provides us a unique fundamental understanding of the ultrafast photoinduced excited-state nonadiabatic process of Cu(I) complexes and their derivatives, which should have potential impacts on various research fields, such as photo-catalysts, dye-sensitized solar cells (DSSCs), and organic light emitting diodes (OLEDs).